1. Field of the Invention
The present invention relates generally to bicycle cranksets and chain rings. More particularly, this invention relates to bicycle chain rings with selectively placed ramps and chamfering for improved shifting performance.
2. Description of Related Art
Conventional bicycle gear systems typically include a crankset including two or three chain rings affixed to a crank arm spider and a separate simple crank arm. The crank arms of a crankset are configured to receive pedals on one end and to be affixed at the other end to a bottom bracket spindle with bearings for rotation. Conventional bicycle gear systems also typically include a rear cog set, occasionally referred to as a cassette or cluster having one or more gears with teeth configured to rotate a rear wheel through a hub with bearing mechanism. Conventional bicycle gear systems further include a bicycle chain which is driven by the chain rings of the crankset which, in turn, drive the cogs of the rear cog set. The gears of the bicycle may be selectively changed using shifters with control wires attached to front and rear derailleurs.
Conventional front derailleurs used with cranksets having two or three chain rings push the chain from one ring to the next using lateral motion. During an up-shift, for example, the chain guide of a front derailleur pushes laterally against the side of a chain until the links of the chain finally catch on a tooth of the larger adjacent chain ring and all subsequent links of the chain follow until the chain is aligned with the teeth of the larger adjacent chain ring. A down-shift is achieved by pushing laterally against the chain resting on the larger chain ring until the chain can fall down to the smaller chain ring.
This conventional method of pushing laterally against the chain with a chain guide provides adequate shifting for most purposes. However, under extreme loading, such as sprinting in the context of racing or out of the saddle climbing, there is a need for quicker shifting, especially up-shifting. A number of solutions have been proposed to improve shifting performance of a front derailleur.
The inventor of the present application has disclosed an improved front derailleur, see e.g., U.S. Pat. Nos. 6,454,671 and 7,025,698, both to Wickliffe, that solves part of the shifting problem by using a chain guide that physically lifts up the bicycle chain during up-shifts and pulls down the bicycle chain during down-shifts, unlike conventional front derailleurs with their predominantly lateral movement of the bicycle chain, during both up- and down-shifts.
Other approaches to improving front derailleur shifting performance have focused on redesigning bicycle chains by shaping outer chain links to more readily grab conventional teeth found on conventional chain rings. By shaping outer chain links of a bicycle chain to bow out laterally or to have chamfered or tapered inner surfaces, such chains may be able to grab chain ring teeth quicker.
Still other approaches to improving front derailleur shifting performance have focused on redesigning the chain rings themselves. For example, U.S. Pat. No. 5,078,653 to Nagano discloses a larger chain ring with selected teeth having reduced height relative to adjacent teeth, i.e., the crests of the selected teeth having been uniformly cut off to reduce height. Additionally, a short pin has been inserted into the inside of the larger chain ring just below the trimmed teeth and opposed to the smaller chain ring. The arrangement disclosed in the '653 patent, facilitates quicker down-shifts by allowing the chain to disengage at the trimmed teeth and be lowered onto the teeth of a smaller chain ring via the short pin. However, there is no indication that the invention disclosed in the '653 patent improves up-shifting, especially during high loads as mentioned above.
U.S. Pat. No. 6,666,786 to Yahata discloses another improvement to down-shifting performance through the use of chamfered chain ring teeth. However, neither the '653 patent nor the '786 patent appear to address, let alone solve, the problem of achieving improved up-shifting.
An approach directed toward improving up-shifting by redesigning a conventional chain ring is disclosed in U.S. Pat. No. 5,413,534 to Nagano. Another approach to improving up-shifting by redesigning the chain rings is disclosed in U.S. Pat. No. 6,572,500 to Tetsuka. The '534 and the '500 patents disclose the use of pins or a pin in combination with a tooth and/or tooth chamfering to aid in up-shifting. However, in both patents the pin or teeth engage a given chain link at the point directly between chain link rollers. The load points of a bicycle chain are at each of the chain link rollers (bushings surrounding pins). Thus, the use of pins as disclosed in the '534 and '500 patents to Nagano and Tetsuka, respectively, may increase stress on the chain especially during high loads and, thus, could lead to increased wear and reduce longevity of the chain.
U.S. Pat. No. 5,876,296 to Hsu et al. discloses the use of an axially oriented recess in combination with a support protrusion to aid in up-shifting. U.S. Pat. No. 5,738,603 to Schmidt et al. discloses a chain ring with pins, chamfered teeth and missing teeth to aid in shifting. Neither of these patents appears to address the added stress to the chain from the allegedly improved up-shifting performance of their respective inventions.
Accordingly, there still exists a need in the art for a bicycle chain ring that achieves improved shifting performance without increasing the stress on the bicycle chain, thereby addressing at least some of the shortcomings of the prior art.